Typically, 3- or more-axle railway bogie designs are provided with at least three rotatable axles, wheels being mounted on axial ends of the axles. Each axle end is supported in rotation by an associated axlebox. Axleboxes are provided with suspension arrangement for the bogie. At least one of the axles forms a middle axle for the bogie designs.
When a 3-axle railway bogie runs a curved track, the middle axle requires an axial displacement with respect to the other two axles that are positioned in the same lateral plan in a curve. Such an axial displacement is possible thanks to axleboxes for the middle axle provided each with special bearing system with axial floating capability.
It is known to provide the axleboxes supporting the middle axle with cylindrical roller bearing units with an axial displacement capability for the rollers. Such bearing units typically comprise a stationary outer ring fixed to an axlebox housing, the housing being fixed to the bogie, a rotating inner ring fixed to the axle, and cylindrical rollers radially housed between the inner and outer rings. A raceway for the rollers is defined in an annular groove provided in the cylindrical bore of the outer ring, the rollers being axially blocked within the groove steps with no possibility to axial move. A raceway for the rollers is defined in an outer cylindrical surface of the inner ring between two steps but, on the contrary to the outer ring, the axial length between the two steps is larger than the axial length of rollers so as to define a running gap. The rollers can axially slide between the two steps of the rotating inner ring and then permits a relative axial displacement between the middle axle and the axlebox housing, hence the bogie.
Typically, the radial space between the inner and outer rings of cylindrical roller bearing units is closed by a sealing arrangement so as to define a rolling chamber for the rollers. The sealing arrangement can consist in shields fixed to the stationary outer ring or housing and forming a labyrinth seal with the rotating inner ring. The sealing arrangement may comprise sealing gaskets made of polymer material and provided with sliding lip. The sealing arrangement prevents any entry of dust, metal particles and water in the rolling chamber. The rolling chamber of cylindrical roller bearing units is provided with lubricant, in particular grease to ensure the lubrication of the rolling and sliding contact between the rollers and the raceways of the inner and outer rings. The sealing arrangement also permits to maintain the lubricant within the rolling chamber.
However, the relative axial displacement between the stationary outer ring and the rotating inner ring induces a grease displacement in the rolling chamber. More precisely, the axial displacement of grease in a closed chamber generates a pumping effect and tends to expel the grease out of the rolling chamber. The sealing arrangement blocks only partly the grease but an important grease leakage out of the bearing unit is generated, degrading the bearing performance and reducing the bearing lifetime.